In its simplest form, a gas turbine installation comprises a compressor, a combustion chamber and a turbine. Intake air is compressed in the compressor, and fuel is then added to it. This is followed by combustion of this mixture in the combustion chamber, with the combustion exhaust gases being fed to the turbine, where their thermal energy is withdrawn and converted into mechanical energy. The compressor is generally driven by the turbine and comprises a multiplicity of compressor blades or vanes.
During compression of the air in the compressor, water may be formed, which under certain circumstances combines with other elements contained in the air to form an electrolyte which can lead to corrosion and erosion at the compressor blades or vanes. To prevent this corrosion and/or erosion, therefore, compressor blades or vanes are generally provided with coatings. Coatings which are particularly suitable in this context are those which comprise a for example phosphate-bonded base matrix with metal particles, such as for example aluminum particles, dispersibly distributed therein. The protective effect of a coating of this type is that the metal particles embedded in the base coating, together with the (more noble) metal of the compressor blade or vane and the electrolyte, from a galvanic cell, in which the metal particles form what are known as sacrificial anodes. The oxidation or corrosion then takes place in the sacrificial anodes, i.e. in the metal particles, and not in the metal of the compressor blade or vane.
The phosphate-bonded base matrix has glass-ceramic properties, is thermally stable, is likewise resistant to the corrosion, and also provides protection against mechanical effects, such as abrasion and erosion.
In addition to the metal particles, the coating may contain further particles as fillers. By way of example, mention may be made at this point of dye particles.
Other types of coatings may be considered as well as phosphate-bonded coatings. EP 0 142 418 B1, EP 0 905 279 A1 and EP 0 995 816 A1 describe chromate/phosphate-based coatings. EP 1 096 040 A2 describes a phosphate/borate-based coating, and EP 0 933 446 B1 describes a phosphate/permanganate-based coating. The coatings described use particle additions with particle sizes of >1 μm. Therefore, the coatings have layer structures with grain sizes of over 1 μm. To obtain a smooth blade or vane outer surface, therefore, a particularly suitable outer coating, known as the top coat, is applied above a primer layer, known as the base coat, of this type.